The invention relates to thermosetting cationic latices useful in coatings and to an improved process for incorporating water insoluble blocked isocyanate crosslinking agents to provide amino-stabilized cationic latices of high cleanliness and stability.
Although water-borne cathodic resin systems are well known, the use of latex binders in cathodic electrocoating is quite new and has not yet achieved full commercial acceptance. Coassigned U.S. Ser. No. 513,621 filed July 14, 1983 (issued as U.S. Pat. No. 4,512,860 on Apr. 23, 1985), which gives background information on electrocoating and especially latex for cathodic deposition, is incorporated herein by reference. The term "latex" is defined as a polymer or copolymer prepared from one or more monomers in an aqueous environment by emulsion polymerization technique. Such latex, usually having an average particle size from about 800 .ANG. to about 10,000 .ANG. and an average molecular weight ranging from about 10,000 up to and above 250,000. Such latex is quite different from the water-reducible or ionizable polymers prepared, not in aqueous medium, but in solvent systems or neat. The latex water-reducible, cation-active polymers have been in commercial use for electrodeposition coatings for some time. The latex polymers and structured cation-active latices of the present invention are preferred over the commercial water-borne or water-reducible coatings. One advantage is improved flexibility and impact resistance. A second advantage is that the coating is more resistant to handling even before baking (Gacesa U.S. Pat. No. 4,175,018). Cationic latices are less stable than their anionic counterpart systems. For electrodeposition coatings, it is necessary to develop a clean latex that can be pumped and will be relatively insensitive to shear forces.
In a commercial operation, the coating is continuously sheared by centrifugal pumping which passes the material through ultrafiltration membranes at a rate of 35-40 gallons/min. Instability of the coating, leading to agglomeration of particles after shear, would cause fouling of the ultrafiltration membrane, application problems, and loss of coating properties. The term "stable cationic latex" or "stable cationic latex paint composition" in the context of the present invention means a latex that when subjected to shear forces and/or pumping will be substantially non-agglomerated as determined by the HB/DCP shear test using a disc centrifuge as described in coassigned application Ser. Nos. 716,664, now U.S. Pat. No. 4,579,889, and 716,665 filed Mar. 27, 1985, and which are incorporated herein by reference. The instant process provides an improved process for incorporation of non-polymerizable blocked isocyanate in a thermosetting latex prepared by emulsion polymerization and overcomes the deficiencies of stability and cleanliness of prior blend systems.
Electrocoating processes and compositions based on cation-active amino-stabilized synthetic latex are known in coassigned U.S. Pat. Nos. 4,512,860; 4,525,260; 4,511,446; and application Ser. No. 723,483 filed Apr. 15, 1985, now U.S. Pat. No. 4,624,762. It is known to crosslink such compositions with external blocked isocyanate crosslinkers such as isophorone diisocyanate blocked with epsilon caprolactam or conventional blocking agents (Column 5, U.S. Pat. No. 4,512,860). Such compositions include crosslinker blends of the blocked isocyanate with preformed latex or with pigment dispersant subsequently mixed with the preformed latex. Coassigned application Ser. No. 723,483 teaches a latex copolymerized with a polymerizable blocked isocyanate where the isocyanate portion becomes part of the polymer chain. U.S. Pat. No. 3,799,854 teaches electrodeposition of a mixture of blocked isocyanate and amino resins which subsequently crosslink on release of the isocyanate on curing.
Coassigned Ser. No. 677,872 filed Dec. 14, 1984, now abandoned, relates to non-latex systems of polymerizable blocked isocyanates polymerized either above or copolymerized with various vinyl unsaturated monomers optionally in the presence of epoxy-amine adducts. Such polymers, dispersed in water by acidification of the amine functionality, are useful for electrocoating metal substrate. These systems differ from the instant invention where the blocked isocyanate is not polymerizable and thus does not become part of the polymer latex chain.
Other art relates generally to polymerization techniques. U.S. Pat. No. 4,039,500 (Bassett et al) teaches a process for controlling molecular weight distribution of latex polymers by using multistep feeding which continuously varies the polymerizable composition where a chain transfer agent is used alone or in conjunction with a crosslinking agent. U.S. Pat. No. 3,562,235 (Ryan) teaches a multistage emulsion polymerization of acrylates to produce thermoplastic polymer. U.S. Pat. No. 4,355,142 (Khungar et al) relates to homogenizing monomer mixes for vinyl latex production. U.S. Pat. No. 3,970,628 (Connelly et al) teaches various thermosetting addition polymers copolymerized with epoxy resin and discusses various feed techniques such as monomer feed, emulsified monomer feed and single feed processes. These systems having acid functionality differ considerably from the amino-stabilized cationic latex of the instant invention.